1. Field of the Invention
The present invention relates to control devices to be installed on a motor vehicle, e.g., an electronic control device of an automatic transmission, which devices are adapted to electronically control various operating states of the motor vehicle.
2. Description of the Prior Art
Control devices for motor vehicles as disclosed in Japanese laid-open Patent Publication Nos. 4-269356 and 5-289949, for example, have been known in the art.
In the control device as disclosed in Japanese laid-open Patent Publication No. 4-269356, modified values stored in RAM are stored into EEPROM through serial communication immediately after a key switch is turned off. The modified values stored in the EEPROM are then re-stored into the RAM immediately after the key switch is turned on. In this manner, the modified values are retained in the EEPROM while the operation of the engine control device is being stopped.
In the control device as disclosed in Japanese laid-open Patent Publication No. 5-289949, data stored in EEPROM are compared with data stored in RAM, and it is determined whether the two data are identical with each other. If the two data are not identical with each other, an operation to delete the data in the EEPROM is performed, and the main routine is finished. The deleting operation has been completed by the time when the next main routine is executed, and new data can be written into the EEPROM. Since the main routine is executed at intervals of 10 ms, data stored in the EEPROM are frequently compared with data stored in the RAM, and the data in the EEPROM are updated. Thus, the latest values are stored in the EEPROM, and the updated data are retained in the EEPROM even after the vehicle is stopped.
The known vehicle control devices as described above, however, suffer from the following problems.
In the control device as disclosed in the Japanese laid-open Patent Publication No. 4-269356, when it is determined that the key switch has just been turned off, the entire region of data that need to be stored is written into the EEPROM, and therefore power needs to be supplied from a backup power supply to a control unit during a period from the time when the key switch is turned off, to the time when all of the data are written into the EEPROM.
In the control device as disclosed in the Japanese laid-open Patent Publication No. 5-289949, data stored in the EEPROM are frequently compared with data stored in the RAM in the main routine executed at intervals of 10 ms, and data in the EEPROM are updated. Thus, the number or frequency of writing is increased where data that are often rewritten or updated must be stored.